refactor: reorganize project into monorepo with frontend scaffolding

Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.

Ref: T007 (specs/001-modbus-relay-control)

backend/src/domain/relay/mod.rs

@@ -0,0 +1,9 @@
+//! Relay domain module.
+//!
+//! This module contains the core domain logic for relay control and management,
+//! including relay types, repository abstractions, and business rules.
+
+/// Repository trait and error types for relay persistence.
+pub mod repository;
+/// Domain types for relay identification and control.
+pub mod types;